1. Field of the Invention
The present invention relates to a rotor core, a motor, and a motor manufacturing method.
2. Description of the Related Art
Conventionally, there is known a so-called inner-rotor-type motor in which a rotary unit having a magnet is arranged inside a stationary unit having a coil. For example, a permanent-magnet-type rotor disclosed in Japanese Patent Application Publication No. H7-312852 or a rotor of an embedded-magnet-type rotary electric machine disclosed in Japanese Patent Application Publication No. 2010-220359 can be applied to the rotary unit of the inner-rotor-type motor.
The permanent-magnet-type rotor disclosed in Japanese Patent Application Publication No. H7-312852 includes a plurality of permanent magnets arranged at a regular interval along a circumferential direction and a plurality of yoke portions provided between the respective permanent magnets, each of the yoke portions having a sector-like cross section (see, for example, claim 1 and FIG. 1 of Japanese Patent Application Publication No. H7-312852). The rotor of a second embodiment of Japanese Patent Application Publication No. 2010-220359 includes a plurality of radially-arranged magnets and a rotor core having a plurality of magnet mounting holes (see, for example, Paragraph 0021 and FIG. 10 of Japanese Patent Application Publication No. 2010-220359).
When the motor is driven, centrifugal forces are exerted on the magnets. For that reason, the permanent-magnet-type rotor disclosed in Japanese Patent Application Publication No. H7-312852 is provided with lug portions protruding into openings of insertion grooves, thereby preventing the permanent magnets from moving toward the outer circumference (see, for example, Paragraph 0006 and FIG. 1 of Japanese Patent Application Publication No. H7-312852). The rotor of a second embodiment of Japanese Patent Application Publication No. 2010-220359 includes outer bridge portions formed at the outer peripheral side of the magnet mounting holes (see, for example, Paragraph 0022 and FIGS. 10 and 11 of Japanese Patent Application Publication No. 2010-220359).
In the rotor in which the magnetic pole portions made of a magnetic material and the magnets are alternately arranged along a circumferential direction, the rotor core becomes partially thin. Therefore, the rotor core is easily deformed by centrifugal forces. If the rotor core is deformed, the magnets are likely to undergo displacement. Deformation of the rotor core and displacement of the magnets tend to pose problems particularly in a high-speed motor.